Avoid unknown warning warnings and fix -Werror on macosx.

clang on macosx does not support -Wunused-but-set-variable; adding the flag
causes additional warnings about the flag. As a more generalized fix, use
-Werror when checking compiler flag support in order to avoid using
unsupported warning flags.

Change-Id: I2529862e211f880d56491eac3b9fa90fff1aa5c3